The present invention generally relates to a method for reducing fouling in refinery units. More specifically, the invention relates to a method for reducing fouling in coking processes and fluid catalytic cracking (FCC).
Fluidized bed coking (fluid coking) and FLEXICOKING are petroleum refining processes in which mixtures of heavy petroleum fractions, typically the non-distillable residue (resid) from fractionation, are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 900xc2x0 F. to 1100xc2x0 F. (about 480xc2x0 C. to 590xc2x0 C.) by heat supplied by fluidized coke particles.
Fouling in the stripper and scrubber sections of a Coker vessel results in decreased capacity and Run length of the unit, culminating in costly unplanned shutdowns.
Fluid Catalytic Cracking (FCC) is another petroleum refining conversion process in which heavy oil, typically the highest boiling distillable fraction, is converted to gasoline, diesel and jet fuel, heating oil, liquefied petroleum gas (LPG), chemical feedstocks, and refinery fuel gas by catalytic decomposition at similarly elevated temperatures of about 900xc2x0 F. to 1100xc2x0 F. (about 480xc2x0 C. to 590xc2x0 C.). Run length or capacity of an FCCU may likewise be limited by deposition of coke in the stripper, reactor overhead, plenum, nozzle, transfer line, or inlet to the fractionator.
What is needed in the art is an efficient, predictable, and effective way to mitigate the fouling tendency in the stripper, scrubber, surge drum and other sections of coking units.
An aspect of the invention is directed to a process for decreasing fouling in refinery unit reactor zones. Said refinery units operate at temperatures of at least about 300xc2x0 C. as measured in the reactor zone. The method comprises preheating a feed to the refinery unit reactor zone and thereafter introducing the feed into the refinery unit reactor zone for reaction. The feed contains polymers and/or oligomers. The feed is preheated for a time and at a temperature sufficient to unzip an effective amount of said polymers and/or oligomers contained in said feed during the preheating step. The preheating step substantially reduces fouling in the refinery unit. Also, the upper limit for the temperature of the preheating is determined to prevent coke formation of more than about 5 percent by weight (wt %).
The invention is particularly useful in coking and cracking refinery units.
Typically, hydrocarbon feeds to refinery coking units, may contain polymers and/or oligomers. These compounds may form in petroleum feeds by thermally initiated oxidative oligomerization reactions with certain feed components and are typically present in amounts of from about 0.02 wt % to about 5 wt % (200 to 50,000 wppm) in the feed. It is believed that these polymers and/or oligomers increase fouling in refinery reactors and equipment. Without wishing to limit the invention in any way it is believed that upon entering the reactor these compounds coat and/or thermally alkylate metal surfaces and the surfaces of any formed coke particles, making them sticky. Sticky coke particles can then agglomerate and also stick to the reactor and related equipment metallic and coke covered surfaces. The feed coating a coke particle surface can undergo further thermal conversion and a mesophase can form at the coke particle contact point, binding the coke particles together. The mesophase formation of the feed between the contact points of agglomeration is believed to be one mechanism leading to cementing together coke particles that were previously held together by weaker adhesive forces.
Typical feeds to coking units may comprise many different polymers, oligomers and mixed polymers and oligomers formed with styrenes, methylstyrenes, indenes, methylindenes and conjugated dienes with minor amounts of indoles, carbazoles, phenols, naphthols, thiophenols, thionaphthols, and the like components. The oligomers/polymers, depending on their molecular weight and the refinery unit reaction temperature, can become sticky and coat or alkylate coke particles with a sticky layer. It is believed that this sticky layer can lead to agglomeration of the coke particles and further coke formation at the contact points of agglomeration before they undergo sufficient thermal cracking conversion to reduce their stickiness via unzipping of the polymer chain.
In typical refinery unit operations, preheat treatments are typically used to raise the temperature of a feed to a desired temperature for energy balance purposes. Once the feed reaches the desired temperature, it is injected through feed nozzles into the reactor, or introduced by other means. It has been discovered that such a preheat scheme is inadequate to unzip any substantial amount of these sticky polymers and/or oligomers found in a feed. Upon entering the reactor the polymers and/or oligomers can lead to increased fouling. For example, thermal alkylation or coating of coke particles by long sticky oligomers and/or polymers may cause coke particle agglomeration. The heavier agglomerated sticky coke particles carry under to the stripper sheds and stick to the sheds causing fouling and buildup of foulant material. Applicants have unexpectedly discovered that preheating can be used to substantially reduce fouling in a refinery unit. Specifically, preheating a refinery feed containing a substantial amount of polymer and/or oligomers, for an amount of time and at a preheat temperature sufficient to unzip an effective amount of said polymers and/or oligomers substantially reduces fouling in the reactor and related reaction zone equipment such as stripper sheds. Without wishing to limit the invention in any way it is postulated that this additional preheating may prevent thermal alkylation and/or coating of coke particles by the most sticky polymeric materials. Alkylation and coating by smaller polymeric units and/or oligomers is then more easily overcome by thermal decomposition reactions at the higher temperature of the coke particles inside the reactor. For example, typically a reactor operating temperature may be about 530xc2x0 C. while the feed may be preheated to about 345xc2x0 C. in the preheat section of the reactor.
Thus, an embodiment of the invention includes preheating the feed to the refinery unit to a bulk temperature compatible with the economics of the unit and additionally continuing to preheat and thereafter introducing the feed into the reaction zone such that the combination of additional preheating and the heating which occurs due to the heating of the feed in the reaction zone causes at least 30%, preferably 50%, more preferably at least about 60%, and most preferably at least about 85% by weight of the polymers and oligomers contained in the feed to unzip. The preheating prior to introduction into the reaction zone is conducted such that no more than about 5 wt % coke is formed in the feed prior to introducing the feed into the reaction zone. The amount of polymers and/or oligomers that unzip is determined by a skilled artisan by running a Thermal Gravimetric Analysis (TGA) on the feed and determining the activation parameters; the preexponential factor and activation energy. The amount of unzipping many include both the amount of unzipping that occurs during preheating in addition to the amount of additional unzipping that would occur during the residence time of the feed in the reactor. For example, to determine if at least about 85% unzipping has occurred, in both the preheating and heating steps, the minimum residence time instead of the average residence time of the feed in the reaction zone would be utilized in the calculation for determining the amount of unzipping. This is because the amount of unzipping in the reactor should only be calculated for the minimum time that the coated coke particle could experience in the reactor section to be certain that it will be dry and not sticky when it enters other areas of the process, e.g., the stripper section.
Preferably, the feed may be preheated at a temperature of at least about 300xc2x0 C. for a time sufficient to cause a substantial amount of polymers and/or oligomers present in the feed which can coat and cause alkylation of coke to unzip. The preheating may be conducted at a temperature of at least about 300xc2x0 C., preferably from about 300xc2x0 C. to about 400xc2x0 C., and more preferably from about 350xc2x0 C. to about 370xc2x0 C. The feed may preferably be preheated for an average residence time of at least about 5 minutes, more preferably for at least about 10 minutes, and most preferably for at least about 15 minutes. Although longer times may be utilized, it is preferable to preheat the feeds for a maximum time of about 1 hour, more preferably for a maximum time of about 30 minutes since longer times impact adversely the economics of the process. Care should be exercised during the preheating step to prevent formation of more than about 5 wt % coke particles in the feed that could precipitate and coat the preheat section walls and result in decreased yield of product and potentially in fouling of the preheat section.
Preheating may be accomplished with any suitable heating means, such as pipes, holding tanks, etc., that provide the required residence time and temperature. Preheat temperatures should preferably be below coke formation temperatures, e.g., about 400xc2x0 C., preferably about 370xc2x0 C. to prevent coke formation in the preheat section, but high enough to degrade or unzip the sticky polymers and/or oligomers. A plot for the cumulative unzipping of the pure polystyrene of MW 230,000 shows that about 10 wt % of the polymer is unzipped every 10 minutes at 348xc2x0 C., S. L. Madorski, xe2x80x9cThe Thermal Degradation of Organic Polymersxe2x80x9d, Interscience Publishers, New York, N.Y. 1963, which is incorporated herein by reference for all purposes to the extent that it is not inconsistent with the present invention disclosure and claims.
Preheating the feed to substantially reduce the amount of polymers and oligomers in the feed as taught herein, may also decrease the viscosity of the feed. This often may be beneficial to the subsequent reaction step, as for example in coking processes wherein generally lower viscosity feeds achieve higher conversions at faster rates because they form thinner and more even coatings on coke particles. Thinner coatings may also favor higher product yields, better product quality and lower levels of bogging of coke particles in the fluidized bed. More even coatings also may prevent buildup of unconverted thick spots that form dry coke at slower rates and make the coke particles more sticky and more susceptible to agglomeration.
The residence time and the temperature of the preheating step required for any given feed may vary. Generally, a preheating residence time and temperature sufficient for a particular feed are those necessary for the polymers and/or oligomers present in the feed, or a portion thereof, to become substantially less sticky or unzipped. One skilled in the art can readily determine the required preheating times and temperatures within the constraints taught herein. For example, it is well known that a clay-gel separation can be run on a sample of the feed to isolate the amount of polymeric material contained therein. A TGA can then be run to determine the weight loss profile. Thus, one skilled in the art would be able to select the appropriate times and temperatures, from the weight loss profile, within the given ranges at which to run the process. Table 1 illustrates the results based on analysis of TGA data for two different samples of polymers/oligomers isolated by clay-gel separation.
The Arrhenius Parameters deduced from TGA data for HCN oligomers were A=1.6xc3x971011 and Ea=40.0 kcal/mole. The Arrhenius Parameters deduced from TGA data for HKGO oligomers were A=4.0xc3x971012 and Ea=42.3 kcal/mole. These A and Ea parameters are lower than those typically associated with petroleum residua thermal cracking decomposition (A=1xc3x971013, Ea=51 kcal/mole) and consistent with polymer unzipping (decomposition/cracking) kinetics. It is clear from the data that preheating for 30 minutes at 360xc2x0 C. will eliminate these species from the feed. Therefore, in these cases, in combination with a higher temperature coking process, preheating at 30 minutes or less would be sufficient.
Once the feed has undergone the preheating step described herein it is introduced into the reaction zone of the refinery unit. The reaction zone which is at about 530xc2x0 C. causes further unzipping of oligomers and polymers at an unzipping rate about 30 to 50 times faster than in the preheat section before the reactor. Thus, since additional unzipping occurs in the reaction itself, shorter preheating times are possible. However, preheating is required to achieve most favorable process heat integration. Additionally, the average residence time of a coke particle in the reaction section is about 10 minutes, but since the residence time of some of the material in the reactor can be only about 10 to 25 seconds, the concern for making sticky coke is alleviated by the additional preheating as taught by the present invention.
Preferably, the preheated feed should not be allowed to cool and sit for extended times because reoligomerization of the decomposition products that are soluble in the feed will likely take place.
The present invention achieves a balance between preheating the feed for a time and temperature sufficient to cause substantial unzipping of polymeric and oligomeric materials to prevent fouling in the reacting zone and not overheating the feed in a manner that would form significant amounts of coke. Preferably, less than 5 wt % coke is formed in the preheating step.
In most cases, the preheating step described herein can be readily accomplished in existing preheat zones of refinery units.
The following examples are meant to be illustrative and not limiting in any way.